It is customary for a camera or other imaging or image processing apparatus to perform some type of exposure control operation. During image capture, for example, an exposure control operation may be performed to match a range of apparent radiances within a scene to a dynamic range of a photosensitive substrate (e.g., photographic film or paper) or image sensor (e.g., a CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) array) within the apparatus. Such an operation may include varying a shutter speed and/or a lens aperture stop of the apparatus.
Exposure control operations according to several different photometric modes are known. One such mode is frame-averaged exposure control, in which the exposure is controlled according to an average brightness level of the scene or image. Another photometric mode of exposure control is center-weighted exposure control, in which the exposure control is weighted according to a brightness level of a center region of the scene or image.
Existing methods of exposure control may produce suboptimal results when the scene being photographed includes one or more people, or when the image being processed is a portrait or other depiction of one or more people. A frame-averaged scheme, for example, may produce an image of a person that is too bright if the person is highlighted or too dark if the person is backlit.
A photograph may appear more aesthetically pleasing when the principal object of interest (e.g., a human face) is positioned off-center. One favored placement of such an object is in accordance with the Golden Ratio: in other words, at a distance from either edge, along the vertical and/or horizontal dimension, of about 61.8% of the size of the image in that dimension. When used with a scene or image that is composed in this manner, a center-weighted exposure control scheme may produce a result in which the principal object of interest is inappropriately exposed.
It is desirable to obtain an image in which a principal object of interest, such as a human face, is appropriately exposed.